1. Field of the Invention
The invention relates to an active matrix liquid crystal display device and more particularly to a liquid crystal display device suitable for a larger screen.
2. Description of the Prior Art
Liquid crystal display devices have the advantages of being thin and lightweight and also allowing low-voltage drive and thus low power consumption. Thus, liquid crystal display devices are used in various types of electronic equipment, such as televisions, PCs (personal computers), PDAs (personal digital assistants), and mobile telephones. In particular, an active matrix liquid crystal display device, which includes a TFT (thin film transistor) which is provided for each picture element (or subpixel) so as to act as a switching element, has high driving capabilities and thus exhibits excellent display characteristics comparable to those of a CRT (cathode ray tube). Thus, active matrix liquid crystal display devices have come into wide use in the fields where CRTs have been heretofore used, such as desktop PCs and televisions.
Generally, a liquid crystal display device comprises two transparent substrates made of thin glass sheets, and liquid crystal sealed in between the substrates. One substrate includes a picture element electrode and a TFT, which are provided for each picture element. The other substrate includes a color filter to be faced with each picture element electrode, and a common electrode common to every picture element. Hereinafter, the substrate having the picture element electrodes and the TFTs will be called a “TFT substrate”, and the substrate to be faced with the TFT substrate will be called an “opposite substrate”. A structure formed of the TFT substrate, the opposite substrate, and liquid crystal sealed in between the substrates is herein referred to as a “liquid crystal panel”.
FIG. 1 is a plan view showing a TFT substrate of a conventional liquid crystal display device.
On a TFT substrate 1, there are disposed a plurality of gate bus lines 3 extending horizontally, and a plurality of data bus lines 5 extending vertically. The gate bus lines 3 and the data bus lines 5 partition the TFT substrate 1 into rectangular regions, which are picture element regions. In each picture element region, there are disposed a TFT 9 which acts as a switching element, and a picture element electrode 8. The gate, source, and drain of the TFT 9 are connected to the gate bus line 3, the data bus line 5, and the picture element electrode 8, respectively. In the case of a transmissive liquid crystal display device, the picture element electrode 8 is made of a transparent electrode such as ITO (Indium-Tin Oxide). In the case of a reflective liquid crystal display device, the picture element electrode 8 is made of any of Al (aluminum) alloys, or the like.
A plurality of gate driver boards 2b are bonded to a first side of the TFT substrate 1 (e.g., the left side thereof in FIG. 1) with ACFs (anisotropic conductive films). The gate driver boards 2b are each made of a flexible printed wiring board, which is formed of a resin film, such as polyimide, and copper foil bonded to the resin film. A gate driver 2a (i.e., a gate driving IC (integrated circuit)) is mounted on each gate driver board 2b. The gate driver 2a has a plurality of output terminals, each of which is electrically connected to the corresponding gate bus line 3.
In the same manner as described above, a plurality of data driver boards 4b, each of which is made of a flexible printed wiring board, are bonded to a second side of the TFT substrate 1 (e.g., the lower side thereof in FIG. 1) with ACFs. A data driver 4a (i.e., an IC for supplying display data) is mounted on each data driver board 4b. The data driver 4a has a plurality of output terminals, each of which is electrically connected to the corresponding data bus line 5.
An input part 10 to be connected to a control circuit board (not shown) is disposed on the TFT substrate 1. Laid on the TFT substrate 1 are wirings 6 which provide connections between the input part 10 and the gate driver boards 2b or between the adjacent gate driver boards 2b, and wirings 7 which provide connections between the input part 10 and the data driver boards 4b or between the adjacent data driver boards 4b. Typically, the wirings 6 and 7 on the TFT substrate 1 are formed simultaneously with the gate bus lines 3 or the data bus lines 5, and the wirings 6 and 7 are each made of, for example, a metal film having a three-layer Ti—Al—Ti structure in which titanium, aluminum, and titanium are stacked in sequence.
In the liquid crystal display device configured as mentioned above, the control circuit board supplies display data, a data clock signal, a gate clock signal, a timing signal, and a predetermined voltage (hereinafter referred to as a “power supply voltage”) to the TFT substrate 1 via the input part 10. The display data, the data clock signal, the timing signal, and the power supply voltage are supplied to each data driver 4a via the wirings 7 disposed on the edge of the TFT substrate 1. The gate clock signal, the timing signal, and the power supply voltage are supplied to each gate driver 2a via the wirings 6 disposed on the edge of the TFT substrate 1.
The data driver 4a outputs the display data to the data bus lines 5 in accordance with the timing in synchronization with the data clock signal within a horizontal synchronization interval. On the other hand, the gate driver 2a outputs a scan signal to the gate bus lines 3 in sequence in accordance with the timing in synchronization with the gate clock signal within a vertical synchronization interval. When the gate bus line 3 receives the scan signal, the TFT 9 for a picture element connected to the gate bus line 3 is turned on, so that the display data supplied to the data bus line 5 is written on the picture element electrode 8. This causes a change in the orientation of liquid crystal molecules in the picture element, thus causing a change in light transmittance of the picture element. Thus, the display data is written on each picture element within a vertical synchronization interval, so that a desired image is displayed on the liquid crystal display device.
Another prior art is disclosed in patent literature 1 (Japanese Unexamined Patent Application Publication No. Hei 09-127540).
In the view of the inventors, the conventional liquid crystal display device, as mentioned above and shown in FIG. 1, has problems as given below. In recent years, there have been demands for larger-sized liquid crystal display devices. However, a larger-sized liquid crystal display device leads to longer wirings 6 and 7 on the substrate 1 and thus to higher wiring resistance. This may cause a relatively significant voltage drop in the wirings 6 and 7, thus causing, for example, variations in color or brightness between picture elements connected to the data bus lines 5 located near the input part 10 and picture elements connected to the data bus lines 5 located far from the input part 10, even though the same display data is supplied to the data bus lines 5. In extreme cases, such an increase in wiring resistance may also make it impossible to supply a predetermined voltage to the gate drivers 2a or the data drivers 4a, thus causing marked deterioration in image display quality.
Patent literature 1, the above publication No. Hei 09-127540, discloses a liquid crystal display device including a TFT substrate having driver ICs mounted thereon, wherein the driver ICs are connected through flexible printed wiring boards (FPC). However, the liquid crystal display device has a disadvantage as given below. Upsizing the liquid crystal display device leads to an increase in the size of the flexible printed wiring board, thus rendering it difficult for an OLB (outer lead bonding) apparatus to bond the flexible printed wiring board to the TFT substrate.